1. Field of the Invention
The present invention relates generally to a semiconductor memory device, and more particularly to a ferroelectric random access memory (FeRAM).
2. Description of the Related Art
A ferroelectric random access memory (FeRAM) stores binary data in a nonvolatile state on the basis of magnitudes of two different polarization modes, making use of the fact that spontaneous polarization, which is one of features of a ferroelectric substance, exhibits a hysteresis characteristic. For instance, D. Takashima et al., “High-Density Chain Ferroelectric Random Memory (CFRAM)” in proc. VLSI Symp., June 1997, pp. 83–84, discloses, in FIG. 4(a), an example of a so-called 2T2C type TC parallel-unit series-connected ferroelectric memory.
As is illustrated in a timing chart of FIG. 4(b) of the above document, when a rewrite (i.e. restore) operation is executed in the FeRAM, the potentials of a pair of plate lines PL and /PL are changed to “H” level and “L” level at each time of data restore. Specifically, as regards a unit cell that stores “0” data, the plate line /PL is once turned off and then an “H” level potential is applied to the plate line PL. As regards a unit cell that stores “1” data, the plate line PL is once turned off, and then an “L” level potential needs to be applied once again to the plate line /PL. Consequently, voltages cannot be applied at a time to the unit cell that stores “0” data and the unit cell that stores “1” data, leading to an increase in restore time.